Door step replacement

[wantafaster1]

Ok, after many months of not being able to face the damn thing I am going to get back onto remaining jobs.

I've bought an M Machine step panel. I'll obviously be trimming off most of the vertical portions as this extends up the A and B post, that's not my area of concern.

What I don't know is how to work the joints on the outside of the car, there's a seam at a 45 degree angle where the step meets the rear quarter, likewise at the A panel. Yes, I can butt these together but can't weld here as I then lose the seam.
I can feel behind the quarter and it looks like there's a third piece of metal lapped over the step and quarter. There's no such thing at the A panel. What do I do, and if possible can anyone show me pictures of how you've done it?